Try turning the phone off and taking out the SIM Card and putting it back in, then turn the phone on and try it again. I've had some issues with phones with SIM Trays like the iPhone or Nexus 5 and the SIM Card not sitting right when you put it in, pulling it out and putting it back in has fixed the issue for me in the past.
Also it can take a little time up to an hour or so for your phone to work after you first activate your SIM card on self serve usually it's it's within 15 minutes though.